1. Field of the Invention
The present invention relates generally to control rods for a nuclear reactor and, more particularly, is concerned with a control rod having an improved construction and improved absorber material arrangement for respectively reducing tensile stress in the control rod and providing a reduced worth control rod tip exposed to each fuel rod of the fuel assemblies associated with the control rod.
2. Description of the Prior Art
It is common practice in nuclear reactors of the boiling water reactor type (BWR) to control the power output and power distribution in the reactor core with control rods insertable from the bottom of the reactor into and from the fuel assemblies in the reactor core. In the BWR, the fuel assemblies each contain a plurality of fuel rods. Each fuel rod provides the nuclear fuel for the reactor in the form of a stack of sintered pellets arranged end-to-end in an elongated cladding tube sealed at its opposite ends. The fuel assemblies are typically grouped in clusters of four with one control rod associated with each four assemblies. In one known design, such as shown for example in U.S. Pat. No. 4,285,769, the control rod has a cruciform cross-sectional shape and a four blade construction. When the control rod is inserted within the fuel assemblies, its blades are positioned in the spaces between the adjacent fuel assemblies. Each such cluster of four fuel assemblies surrounding a control rod is commonly referred to as a fuel cell of the reactor core.
In a BWR control rod having the cruciform cross-sectional configuration, boron carbide powder is commonly used as the neutron absorber material. It is vibratory compacted into small diameter stainless steel tubes which are held in a cruciform array by a stainless steel sheath of each of the control rod blades. In operation, the control rods are moved vertically downward by a hydraulic drive mechanism in incremental steps, such as six inches in length, to increase reactor power.
Control rods of this composition have two undesirable characteristics: (1) stainless steel stress corrosion cracking (SCC); and (2) pellet clad interaction (PCI). On the one hand, SCC results from relatively high tensile stresses in the stainless steel tubes containing the boron carbide powder. The tensile stresses are produced by several conditions: (1) consolidation of the vibratory compacted boron carbide powder, (2) internal pressure due to irradiation-induced helium generation; and (3) irradiation-induced swelling of the powder. SCC frequently results in split tubes with loss of boron carbide powder from the control rods. On the other hand, PCI is caused by a step increase in power in the fuel pellets within the cladding tubes adjacent to the tip of the control rod. PCI often leads to fuel rod clad perforations. A power increase of as much as 150 percent at the tip of the corner fuel rods adjacent to the control rod can be expected. This sudden increase in pellet power results in an increase in pellet temperature which causes it to expand radially outward into contact with the fuel rod cladding. The resultant cladding strain, along with chemical attack on the inside of the cladding, results in PCI.
One method of alleviating the PCI problem is to restrict control rod movement during high reactor power. Power is reduced by flow control before the control rods can be moved. A second method is to line the inside of the fuel rod with a material which will tend to prevent PCI. Both of these methods are closely in terms of reactor availability and fuel fabrication costs.
The solution proposed to alleviate PCI in the control rods of the above-mentioned patent is to produce a "grey tip" on the end of the control rods by providing the vertical tubes containing the boron carbide powder with shorter lengths at the end of the blades or by using short sections of hafnium at the blade tips. Thus, the grey tip is composed of full length absorber material at the center of the cruciform-shaped control rod with gradually shorter lengths progressing outward on the blades. From a macroscope standpoint, this can be considered as a "grey tip," but from an individual fuel rod standpoint, it is not. When the control rod is moved, the pellets in the corner fuel rods of the fuel assemblies see the same large step change in reactivity (i.e. neutron flux) as in a standard control rod without any grey tip since the absorber tubes at the center of the control rod are the same diameter and are filled to the top with absorber material. Since the corner fuel rods are the limiting rods from a PCI standpoint, the design of the above-cited patent does not improve the PCI problem. Additionally, this design does not compensate for the SCC problem in the control rods. Tensile stress caused by helium gas pressure buildup due to the absorption of neutrons by the boron atoms and caused by swelling of the boron due to the same interaction can occur in this design. Boron carbide will leak out of the control rod and thereby limit the life of the rods.
Consequently, a need exists for an improved control rod design which will alleviate both the SCC problem and the PCI problem. Such design can be incorporated into control rods used for replacement of the original rods which have a limited lifetime.